1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring curl of a web which is fed in its longitudinal direction, a method of and an apparatus for correcting curl of such a web, and an apparatus for cutting such a web.
2. Description of the Related Art
Generally, presensitized plates for planographic printing have a support layer coated with a photosensitive material and comprising a thin metal sheet of aluminum, for example, which has a thickness ranging from 0.1 mm to 0.5 mm, for example. A web cutting apparatus for cutting a web into such thin metal sheets for use in presensitized plates has a reel which supports a roll of web. The web unwound from the web roll on the reel is corrected out of curling into a flat shape by a curl correcting apparatus such as a decurling unit or a roller leveler. The decurled web is then trimmed by a slitter and cut by a shear into sheets which are delivered by a feed mechanism such as a conveyor to a stacker section where they are stacked.
In the web cutting apparatus, sheets cut from the web are inspected for curl or shape defects. For example, sheets cut by the shear are sampled on a production line, and the sampled sheet is measured in an off-line area for curl or shape defects (hereinafter referred to as "prior art 1"). Japanese Laid-Open Patent Publication No. 6-50751 discloses a process for detecting the amount of a bend (curl) in the leading end of a cut sheet at the exit end of a conveyor as a feed mechanism (hereinafter referred to as "prior art 2").
According to the prior art 1, a sheet on the production line is removed by a sampling unit and then measured in the off-line area for curl or shape defects. The measuring process requires a considerable amount of manual work which is time-consuming. Since the production line is shut off until the measuring process is finished, the production line is poor in efficiency.
According to the prior art 2, the amount of a bend in the leading end of a cut sheet is detected after the sheet is delivered to the exit end of the conveyor. As a plurality of defective sheets may possibly be present between a curl correcting unit and the exit end of the conveyor, the yield of normal sheets is relatively poor.
Settings for a curl correcting unit for correcting curl in sheets are changed usually depending on the experience of the operator. Therefore, the number of sheets which are rejected as being defective varies from operator to operator. The web wound around the reel has different amounts of curl at the outer periphery of the roll than near the reel of the roll. The settings for the curl correcting unit must be different at the outer periphery of the roll than near the reel. The settings for the curl correcting unit for decurling the web in the vicinity of the reel are changed based on the experience of the operator. This practice imposes an undue burden on the operator. If settings for the curl correcting unit are changed inappropriately, then cut sheets tend to become tortuous on the conveyor and to be stacked improperly. As a result, the yield of proper sheets is poor, and the efficiency of the production line is low.
When another web unwound from a web roll on the reel is cut into sheets for use in new presensitized plates, the curl correcting unit is first operated with provisional settings. Then, the provisional settings are modified into final settings based on the results of an inspection process conducted on the sheets for curling. Until the final settings are determined, the production line is operated with the provisional settings, which results in the possibility of a low product yield. To avoid the possibility of a low product yield, it is necessary to operate the production line at a low speed until the final settings are determined. As a consequence, the rate of production of sheets for use in presensitized plates is relatively low.